Means for producing and amplifying optical energy

ABSTRACT

A laserable material with a host of non-gaseous, non-periodic atomic structure is provided. The host material is plastic and dispersed in solid solution within the plastic is a chelate of a rate earth metal. The material exhibits narrow-line fluorescence when excited by a high energy light source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 168,012, filedJan. 16, 1962, which is a continuation-in-part of application Ser. No.148,204, filed Oct. 27, 1961.

This invention relates generally to optical masers, or lasers as theyare sometimes called, and related light-generating and light-amplifyingdevices.

The word laser is being used in the instant disclosure in preference tomaser since devices and components of this invention are used in thelight or optical region of the electro-magnetic spectrum rather than inthe microwave region thereof; it being appreciated, of course, that thename "MASER", now in common use, was originally derived from theinitials of the title Microwave Amplification by Stimulated Emission ofRadiation. With the word LIGHT substituted for MICROWAVE, the word"MASER" becomes "LASER". Accordingly, the terms optical maser and laseras used herein are equivalents.

More particularly, the invention relates to improvements in laserdevices and components employing special glasses, utilizing known glasscompositions in which suitable maser materials are incorporated orconfined, as the laser materials and which devices and componentsbecause of their composition and construction are able to provide newand improved operating conditions and efficiencies not previouslyobtained by any optical maser or laser or equivalent devices of knownconstructions.

The broad general principles of operation of optical masers, or lasers,are well-known, and several different solid state laser materials, forexample natural or artificial rubies, have already been discovered. Itshould be appreciated, however, that each of these earlier solid statelaser materials has been a crystalline material and, accordingly, eachhas been accompanied by limitations as to its construction, operativeabilities, response characteristics and the like, as will appear fromthe description which follows.

It has now been found that a laser material or laser component may beformed from any one of a plurality of specially prepared glasses orglassy materials and when properly made and operated will provide newoperating results not heretofore obtainable by any known laser materialor device.

The terms glass and glassy material as used in their broader sensesthroughout this disclosure, it should be noted, are intended to includeboth organic and inorganic rigid materials which are of a plastic-likenature having a non-periodic atomic structure, as distinguished frommaterials having their atoms in an orderly periodic array, and wouldinclude not only commercially available inorganic glasses such assoda-lime glass, barium crown glass, flint glass, lead glass, arsenictrisulphide glass and the like but also certain commercially availableplastics, glycerine and other devitrified sugar substances, and mixturesor organic solutions which, when cooled, form clear glasses such asE.P.A. (ether, isopentane and alcohol).

In fact, it has been found that when a suitable active material such asany one of certain materials selected from the rare earth group or otherspecified materials is combined with such a glass and used in the mannerhereinafter disclosed, advantageous results in light generation andcoherent light amplification will be obtained. The term light as hereinused is intended to cover the approximate wave length range from about 2× 10³ Angstroms to about 10 × 10⁶ Angstroms.

Several different solid state laser materials including red ruby, pinkruby, divalent samarium in calcium fluoride, trivalent uranium in bariumfluoride and trivalent uranium in calcium fluoride have already beendiscovered and used to demonstrate lasering action. It should be noted,however, that each of these laser materials is in a crystalline formand, accordingly, is very much limited in size, with the result thateven though laser action or the like has been obtained therewith undercertain operative conditions, nevertheless, the performance and possibleuses of these materials are limited.

On the other hand, by following the teachings of the present inventionwherein a specially prepared glass or glassy material embodying orconfining suitable active or laser materials therein are provided, it ispossible to obtain results not obtainable by any of the solid statelaser materials mentioned above. For example, laser components formed ofglass may be compounded and worked or shaped in many different knownways and techniques. Furthermore, such new glass laser materials may beformed into thin, long rods or fibers and easily controlled as to exactthickness and cross-sectional shape or molded into preferred resonantcavity shapes or the like, thereby making it possible to obtain many newresults and advantages, later to be more fully described, and whichresults and advantages were not obtainable by any of the earlier knownsolid state laser materials.

It is, accordingly, an object of the present invention to provide novellaser components, laser devices, laser assemblies and the likecomprising a specially prepared glass embodying a suitable predeterminedamount of laser material.

It is an additional object of the invention to provide a laser componentand the like of the character described and which component is formed ofglass embodying an active laser ingredient selected from the groupincluding Praseodynium ⁺ ⁺ ⁺, Neodymium ≲⁺ ⁺, Samarium⁺ ⁺, Samarium⁺ ⁺⁺, Europium⁺ ⁺, Europium⁺ ⁺ ⁺, Uranium⁺ ⁺ ⁺, Terbium⁺ ⁺ ⁺, Holmium⁺ ⁺ ⁺,Erbium⁺ ⁺ ⁺, Thulium⁺ ⁺ ⁺, Dyprosium⁺ ⁺ ⁺, Ytterbium⁺ ⁺ ⁺, and Cerium⁺ ⁺⁺, and which laser component may be readily formed into a suitablepredetermined shape so as to efficiently function in a predeterminedmanner as a laser oscillator or as a light amplifier.

Another object of the invention is to provide a laser component formedof glass and comprising an ingredient selected from the above-mentionedgroup and with the weight of said ingredient being between 0.1% and 30%of the weight of said glass.

It is also an object of the invention to provide a resonant cavity laseror the like formed of glass and of such construction as to have a highquality factor or Q for the laser oscillations.

Another object of the invention is to provide laser means formed ofglass and so constructed and arranged as to function as a travellingwave type of laser amplifier, such as might be used with a communicationsystem using optical energy as the signal.

It is also an object of the invention to provide laser means formed ofglass and arranged so as to function as a broad band amplifier forsignals in the optical region of the spectrum.

Other objects and advantages of the invention will become apparent fromthe detailed description which follows when taken in conjunction withthe accompanying drawings in which:

FIG. 1 is a diagrammatic view, partly in cross-section, showing a laserassembly including a rod formed of glass embodying an active lasermaterial;

FIG. 2 is an enlarged fragmentary view of one end portion of the rod ofFIG. 1;

FIG. 3 is a view similar to FIG. 1 but showing a modified form of laserconstruction;

FIG. 4 is a diagrammatic view, partly in section, showing a travellingwave type of laser amplifier embodying the invention;

FIG. 5 is another modified form of a laser construction;

FIG. 6 is a perspective view, partly in section, of another modifiedform of laser component;

FIG. 7 is a diagrammatic showing of portions of a communication lineemploying optical energy as the signal and laser amplifier means atselected points as booster means;

FIG. 8 is a graph showing, as an example, the fluorescent spectrumobtained from a neodymium glass laser when laser oscillations areexcited and when there are no laser oscillations, that is, only when thenormal spontaneous emission occurs; the non-laser curve was obtained byreplacing the globar source in a Perkin-Elmer single beam infraredspectrometer with a sample of the neodymium glass and by illuminatingthis glass with visible light only;

FIG. 9 is a diagram showing the energy levels, for example, of theneodymium⁺ ⁺ ⁺ ion and the levels between which spontaneous emissionoccur to give the normal fluorescent spectrum for neodymium in theglass;

FIG. 10 is a graph showing the absorption spectrum of the neodymium ion;

FIG. 11 is a perspective view, partly in section, of another form oflaser component; and

FIg. 12 is a cross-sectional view of a laser structure embodying afurther modified form of the invention, FIG. 12a representing anenlarged cross-sectional view of the hollow clad fiber used in the FIG.12 construction.

Referring to the drawings in detail and particularly FIG. 1, it will beseen that the numeral 10 indicates generally a resonant-cavity form oflaser assembly for generating laser emission. This laser assemblycomprises a laser component 12 in the form of a thin, long,cylindrically-shaped glass rod about which is concentrically disposed inspaced relation thereto a spirally shaped flash tube 14. The flash tubeis of known construction and has electrical leads 16 attached to itsopposite ends and which are also connected to a conventional highpotential source of electrical power 18 in known fashion. Encircling toconvolutes of the flash tube 14 and disposed so as to be in closelyadjacent relation thereto is a conductor 20 for triggering the flashtube by a high voltage pulse as desired, said pulse being provided bythe control unit 22.

About the flash tube 14 and laser rod 12 and in concentric relationthereto is shown a hollow cylindrically-shaped member 24 which has itsinternal surface adapted and arranged to act as a light reflector. Ifreference is now made to FIG. 2, it will be seen that the laser rod 12,in its embodiment of the invention, comprises a thincylindrically-shaped fiber 26 formed of a barium crown glass containingan active laser material or ingredient. This fiber 26 is centrallydisposed within an appreciably thicker cladding 28 formed of commoncommercially available soda-lime crown glass which is preferably of alesser refractive index than the refractive index of the barium crownglass forming the fiber 26. Solid rods of the laser glass without thecladding 28 may also be used.

A clad fiber of the type just described may be conveniently fabricatedin the manner disclosed in the U.S. Pat. No. 2,992,517, eitherappropriately changing the contour of the forming rolls shown in thispatent where the laser rod 12 is to have circular cross-section asherein shown by way of example or using cylindrical forming rolls asshown in the patent where the laser rod is to have a rectangularcross-section. The laser rod 12 may also be fabricated, especially wherea relatively rigid rod structure is desired, according to the method andby use of the apparatus disclosed in the U.S. Pat. to Hicks, Jr. No.2,980,957. The method of the latter patent uses a hollow tubing of thecladding glass into which is inserted a solid rod of the laser glass.The assembly of tubing and rod, carefully cleaned prior to assembly, isinserted vertically into a furnace having three successively arrangedheating zones and the upper end of the assembly is held by a clamp whichmoves slowly down through the furnace as the drawing operation proceeds.By conventional use of a baiting rod, the heat-softened lower end of thetubing-rod assembly is initially drawn out through an aperture in thebottom wall of the furnace at a velocity so related to the severalfurnace temperatures that the cladding-rod assembly is reduced to adesired external diameter. The drawing velocity is preferably maintaineduniform by use of a vertical straight-line-draw machine having a screwdriven clamp attached to the baiting rod or using traction drawing rollsengaging the drawn laser rod. Laser rods of rectangular cross-sectionare conveniently fabricated by use of forming rolls as shown in thelast-mentioned Hicks, Jr. patent, but the use of forming rolls ispreferably dispensed with in fabricating laser rods of circularcross-section.

To obtain laser oscillations in a resonant cavity structure requires theestablishment within the cavity of standing waves. This can be doneessentially in three ways as follows.

One method of doing this is to have optically homogeneous material withtwo reflectors facing each other and disposed parallel to one anotherand with the optically active material contained between them. Thestanding waves herein established correspond to approximately planewaves which reflect back and forth between the reflectors. In order tohave good laser operation in such a cavity, a high degree of opticalperfection is required. This is difficult to obtain in crystals,especially large crystals. However, by having the active material in theform of a glass, as disclosed herein, samples of glass of large sizescan be made with superior optical qualities for use in forming lasers inaccordance with the present invention.

The second method for obtaining standing waves is dependent on a surfacewave which is established at the interface between two materials ofdiffering indices of refraction -- between any laser material andanything else -- at least one or both of which materials contains activeions suitable for laser oscillations. The direction of propagation ofsuch a surface wave is parallel to the interface. The resonant cavity isformed by terminating the interface with two reflectors which face eachother and are substantially perpendicular to the interface. The use oftwo glasses of different indices of refraction makes it possible to formsuch a surface of superior optical quality.

The third method is a small fiber exciting distinct modes. Suitable modepropagation fiber constructions are disclosed in the copendingapplication of John W. Hicks, Jr. et al. entitled OPTICAL ENERGYTRANSMITTING DEVICES AND METHOD OF MAKING SAME, Ser. No. 12,128, filedMar. 1, 1960, and the copending application of Elias Snitzer et al.entitled OPTICAL RESONANT CAVITY, Ser. No. 66,815, filed Nov. 2, 1960and both assigned to the same assignee as the present application. Thesefiber constructions are similar to that illustrated in the enlarged endview of FIG. 2, and include a cladding of a low index of refractionglass surrounding a core of high index of refraction glass. As thediameter of a core of circular cross-section is reduced, modepropagation can be limited to one or a few low order modes. The lowestorder mode, the HE₁₁ hybred mode, does not have a cut-off. The cut-offparameters for the modes designated TE_(0m), TM_(0m), HE_(1m), andEH_(nm) (n≧1) are the mth roots of the nth order Bessel function, i.e.,

    J .sub.n (u.sub.nm) = 0,                                   (1)

and for the HE_(nm) (n≧2) modes it is given by the solutions of ##EQU1##where n₁ and n₂ are the indices of refraction of core and cladding, Thecutoff parameters are in turn related to the properties of the guide by

    u.sub.nm =  2 π(a/λ)(n.sub.1.sup.2 - n.sub.2.sup.2 ).sup.1/2, (3)

where a is the radius of the core, and λ the free space wavelength.

Except for the n=0 modes, for each mode another one can be obtained withthe same propagation properties by rotating the field distribution byπ/n. Hence, all the modes with n≧1 are doubly degenerate. Thisdegeneracy can be removed by destroying the circular symmetry, bydistorting for instance, the fiber cross section from a circle into anellipse. By reducing the diameter of the core, only the HE₁₁ can beallowed to propagate. The next higher modes are the TE_(0m) and TM_(0m)with u₀₁ = 2.402. For an index of refraction combination of core andcladding of 1.56-1.52, the cutoff for the 01 modes for the green line ofHg at 0.546μ corresponds to a radius of 0.59μ. This is well above the0.1μ radius core of fibers that have been made and in which the HE₁₁mode has been observed in the visible spectrum.

As will presently be explained more fully, the centrally disposed fiber26 of the FIG. 2 laser rod construction is formed of a lasering materialand provides a lasering operation giving rise to induced emission lightenergy within the core material 26. The portion of the emitted lightenergy which propagates by mode selection along the length of the corefiber 26 induces or stimulates further light emission, thus enhancingthe lasering action. Since the probability for induced emission by onephoton in a given mode is the same as the probability for spontaneousemission into the same mode, a measure of the coupling into a desiredmode is given by the fraction of spontaneous emissions into that mode.

In an open structure like a Fabry-Perot Interprometer (FPI), spontaneousemission is equally probable into all the modes, both the desired onescorresponding to normal or near normal incidence of radiation reflectedback and forth between the FPI plates, and for the light which isemitted out the sides.

For a Lorentzian line shape with line width Δλ, the number of modes in avolume V is

    P (λ)Δλ=8π.sup.2 VΔλ/λ.sup.4. (4)

where P is the density of modes per unit wavelength interval without theline of Lorentzian line shape and of line width Δλ centered about thewavelength λ. The value of V depends on the design of the FPI. For arough estimate let V=1 cm³, λ=0.6 μ and Δλ=10.sup.⁻² Angstrom. Then thefraction of all spontaneous emissions into one mode is approximately10.sup.⁻⁹. To initiate maser oscillations, enough pumping power must besupplied to overcome the substantial relaxation mechanism of spontaneousemissions into undesired modes.

Mode coupling in a dielectric waveguide of the FIG. 2 type isintermediate between that of an open structure such as an FPI and aclosed structure like a metallic waveguide. A radiating atom in the coreof a fiber can emit into either a bound dielectric waveguide mode or anyone of a large number of unbound modes. From a geometrical opticsviewpoint, the unbound modes correspond to light emitted in the corefiber leaving the fiber by striking the core-cladding interface at lessthan the critical angle for total internal reflection. If the unboundmodes are included, the total number of modes is approximately the sameas for the FPI. However, the coupling into a bound mode is much strongerthan for the unbound ones.

The dielectric guide of FIG. 2 is similar to a metal guide with wallsthat have a finite value of electrical conductivity. In the latter casethere is a large number of nonpropagating modes which are stronglyabsorbed. The coupling into the weakly absorbed propagating modes ismuch stronger than for the other modes. In the limit of infiniteconductivity for the walls, the propagating modes are undamped and thecoupling to the others is zero. To obtain emission only into boundwaveguide modes in the dielectric guide would require a core index ofrefraction which is infinitely large compared to the cladding index. Theconductivity of the walls of a metallic guide is usually sufficientlyhigh to justify considering it as a single or few mode structure.However, in the dielectric guide all the modes must be considered,because the core index is far from infinitely large compared with thecladding. In fact, for glass fibers in the visible region of thespectrum, the refractive indices differ by only a few percent.

For an emitting atom in the core, the coupling into modes in which thelight propagates out the sides is less than for an open structure. Thiscan be seen by considering the reverse process of light incident on thecore from outside. Owing to the difference in refractive indices of coreand cladding, partial reflection occurs. Hence the unbound modes have anaverage energy density in the core which is smaller than in thecladding. However, the index difference is small for glass fibers, and asufficiently good approximation is obtained by assuming that thecoupling to the unbound modes is the same as for an open structure.

In the nonrelativistic limit, the matrix element for a transition fromstate a to state b of an atom with the spontaneous emission of a photoninto the sth mode with vector potential whose spatial part is A_(s) (r)is proportional to

    ∫ψb*(p.A.sub.s)ψa.sup.d.sup.τ,            (5)

where dτ is the incremental volume and p is the momentum of the electronmaking the transition from eigenstates ψa to ψb. For the unbound states,A_(s) can be approximately taken as plane waves in a volume L³,

    a.sub.s = e.sub.s (4πc.sup.2 /L.sup.3)1/2 exp {ik.sub.s .r}, (6)

where c is the velocity of light, e_(s) is a unit vector in thedirection of polarization, and k_(s) the propagation vector of theemitted photon. The number of unbound states is given by Eq. (4). Thetransition probability T₁ for spontaneous emission into all the unboundmodes is proportional to the product of the square of the matrix elementfor a transition into one of the unbound modes times the number of modesper unit frequency. Hence,

    T.sub.1 α 8π.sup.2 n.sub.a.sup.2 /(cλ.sup.2), (7)

where n_(a) = (n₁ + n₂)/2 is the average index of core and cladding. Ithas been included to take approximate account of the wavelength in therefractive medium of which the fibers are made.

For the bound modes the transition probability depends on an integral ofthe form of Eq. (5), with A_(s) now the vector potential for a boundmode. It is sufficient for the approximate treatment here to take A_(s)of the form of Eq. (6) but with the volume L³ replaced by LA, where L isthe length of the fiber and A the core area. The transition probabilityT₂ into the bound modes is then proportional to 1/LA times the densityof modes in a fiber of length L. If the fiber cross section is smallenough so that only the doubly degenerate HE₁₁ mode propagates, thenumber of modes in length L for a linewidth of Δλ with a Lorentzian lineshape is

    4 πL(Δλ)n.sub.a /λ.sup.2.           (8)

The average index n_(a) is used instead of n₁ because the phase velocityfor mode propagation is intermediate between c/n.sub. 1 and c/n₂ . Theexact value depends on how far λ is from the cutoff wavelength for thatmode. Then T₂ becomes

    T.sub.2 α4 π /cA.                                 (9)

the ratio of the spontaneous emission probability into the desired boundmodes to the probability for emission into all the unbound ones is

    T.sub.2 /T.sub.1 = λ.sup.2 /(2π An.sub.a.sup.2). (10)

The condition for only HE₁₁ propagation is just met if Eq. (3) issatisified with u₀₁ = 2.402. Using this value in Eq. (3), the area canbe eliminated in Eq. (10) to give finally

    T.sub.2 /T.sub.1 =  1.4(n.sub.1 -n.sub.2)/(n.sub.1 +n.sub.2). (11)

The above gives the fraction of light emitted into the two HE₁₁ modeswhich are polarized perpendicular to one another. For the index ofrefraction combination of n₁ =1.56 and n₂ =1.52, T₂ /T₁ is approximately1.8×10.sup.⁻².

A fiber terminated with at least partially reflecting ends provides aresonant cavity structure. To find the ratio of the spontaneous emissionT₂ ¹ into a single cavity mode to that emitted into all the unboundmodes two cases must be distinguished. They occur when the materiallinewidth Δλm is greater or less than the cavity linewidth Δλc. ForΔλm >> Δλ c one need only divide Eq. (10) or Eq. (11) by the number ofmodes given by Eq. (8). From the definition of the quality factor Q asderived below, the resulting equation is unaltered by multiplication byQ(Δλc)/λ . This gives

    T.sub.2.sup.1 /T.sub.1 =[ Q λ.sup.3 /16π.sup.2 ALn.sub.a.sup.3 ].[ 2Δλc/Δλm] .                 (12)

From a detailed quantum mechanical calculation by Senitzky appearing inthe Phys. Rev., Vol. 119, page 1807 (1960), the second factor in Eq.(12) is replaced by one in which αλc>>Δλm. In this case the usualPurcell formula [Phys. Rev., Vol. 69, page 681 (1946)] is obtained forthe enhancement of spontaneous emission in a cavity.

If N is the excess of atomic or molecular systems in the upper of thetwo states between which maser action takes place, the condition foroscillation in a fiber of volume V is

    N ≧ VhΔv/(4πμ.sup.2 Q),                 (13)

where h is Planck's constant and Δ v and μ are, respectively, thehalf-width and matrix element for the transition.

The quality factor or Q (ratio of energy stored to energy dissipated percycle) is determined by the bulk absorption in the glass fiber and thereflectivity r of the end plates. In fibers of 0.002-in. core diametermade from good optical quality glass, the attenuation is approximately50% in seven feet. From other observations on large fibers, theindicators are that some of the loss is due to scattering byinhomogeneities at the boundary between core and cladding. Since theinhomogeneities are drawn out over a longer length in the smallerfibers, the transmission in this case is at least as good as the figurequoted above.

For reasonable values of the reflectivity of the ends, say r=0.90 tor=0.98, and for lengths L of a few centimeters, the Q is determined byr. Its value is given approximately by

    Q = 2 π(L/λ)(1-r).sup..sup.-1                    (14)

If the difference in energy of a pumping photon versus the emitted maserphoton, and other modes of decay are neglected, the minimum power tosustain N excited systems is

    P = Nhv/τ                                              (15)

where hv is the energy of one photon and τ is the lifetime of theexcited state. Due to the dependence of N on V in Eq. (13), both N and Pare reduced by the small cross section of the fiber. This is to beexpected, since maser action depends primarily on the densities of pumppower and excited states and not on their absolute values.

It will be apparent from the foregoing mathematical analysis that alaser element or component embodying the present invention has theimportant advantage that the external light source need provide lesspump power per unit area at the lasering fiber 26 than in the case ofthe FPI structure since the volume of lasering material per unit lengthis less in the laser element of the invention than in the FPI type ofstructure. Accordingly, the present invention enables lasering action tobe attained by use of a light source of relatively high intensity but ofrelatively low total power as contrasted to the FPI type of structure.At the same time, the quality factor or Q of a resonant structureutilizing a lasering fiber embodying the present invention is notimpaired and may even be substantially enhanced as compared with the FPItype of resonant structure since, as indicated by Eq. (14), the Q isdirectly proportional to the length L of the lasering fiber.Furthermore, as noted above, a clad lasering structure embodying thepresent invention is characterized by good mode selection of the emittedlight energy and accordingly there is substantially stronger modecoupling to the desired modes as contrasted with the FPI type ofstructure.

Different types of propagation modes for laser oscillations have beenobserved in the neodymium glass laser described herein; in largecross-section rods of approximately one-quarter inch diameter and threeinch length having the sides ground but not polished but with the endsparallel and polished and fabricated of uniform optical materialterminated by reflectors, as in a Fabry-Perot type resonant cavity,various laser oscillation modes have been observed. For clad rods, themodes corresponding to the clad dielectric wave guide modes abovediscussed have been observed and there have also been observed surfacewave type modes in which the material lasers in a filament that stradlesthe material of the core and cladding.

A further advantage of substantial importance and heretoforeunobtainable is that of being able to draw long clad fibers of superioroptical quality different indices with the core or cladding or bothcontaining the active ion which is suitable for laser oscillations. Inaddition to making single, long clad fibers, one can also make largefused masses of fibers with the cores substantially parallel endseparated from one another by a good optical quality glass which servesas the cladding and which is of a lower index of refraction than thecore. Such fiber assemblies per se, but without use of a laser material,are now known in the prior art and are referred to as fused fiber opticassemblies. An assembly of this nature but using clad laser fibers maybe fabricated in the manner disclosed in the U.S. Pat. No. 2,992,516 toF. H. Norton.

By controlling the size and indices of refraction of the core andcladding in the pumping wave lengths as distinct from the wave length atwhich laser oscillation takes place, that is, by using glasses for thecore and cladding that have different dispersions, it is possible toobtain large assemblies of glass fibers which present essentiallyuniform optical characteristics for light travelling down the axis ofsuch a bundle but which presents essentially different opticalproperties to the pumping light incident from the side thereof.

In forming such bundles, there are many commercially available glassesthat have the same index of refraction but whose dispersions aredifferent, and thus many different commercially available combinationsof glasses could be used. For example, the commercially available SchottGlass No. F16 has an index of refraction at 480 millimicrons of 1.60546and at 656.3 millimicrons has an index of refraction of 1.58789, whereasSchott Glass No. SK13 has an index of refraction of 1.59947 at the wavelength 480 millimicrons and an index of refraction of 1.58873 at 656.3millimicrons. If the SK13 is used as the core glass of a clad fiber, andat the wave length of approximately 1.06 microns at which the laseroscillations take place in this glass when doped with an appropriateamount of neodymium as hereinafter explained, such a fiber can readilybe excited in desired wave guide modes by appropriate selection of thecore fiber dimensions as previously explained in connection with modepropagation. In this case, the F16 glass would be used as the claddingand a fused array or bundle of such clad fibers would present differentoptical properties to the pumping light since the indices of refractionof the core and cladding are essentially the same in the center of thevisible region of the spectrum whereas at the blue end of the spectrumthe core index of refraction is less than that of the cladding. Thispermits the optical properties for the fused fiber bundle to becontrolled independently for the side-incident pumping light and for thelaser light which propagates parallel to the laser fibers.

In a particular laser rod construction of the type shown in FIGS. 1 and2, the laser component comprising the fiber and cladding was in the formof a rod approximately three inches in length and 1/8 of an inch indiameter. The fiber 26 was formed of barium crown glass having acomposition hereinafter described and containing as the active laseringingredient the rare earth material trivalent neodymium, and wasapproximately 0.015 inches in diameter. In the preparation of this lasercomponent, the opposite ends of the straight rod 12 were ground andpolished so as to be in substantially parallel relation to each otherand normal to the axis of the rod (good to within one minute of arc) andwere thereafter coated with a silver layer, one of which is shown at 30,so as to be highly reflective to light within the fiber and impingingthereon. The exposed surfaces of the silver layers, such as the surface32, were thereafter coated with a very thin protective coating ofmagnesium fluoride. Furthermore, the application of the silver layer onthe end of the rod shown in FIG. 2 (which is the exit end of the rod)was so controlled as to provide approximately a two percent transmissionfactor. Thereafter, small protective caps 34 and 36 of a reflectingmetal such as aluminum were applied over the ends of the rod, the cap 34having a solid end wall while the exit-end cap 36 was provided with anopening 36a. The protective end caps are used to prevent the flash-tubelight from deteriorating the reflective silvered ends. If in aparticular construction the laser rod should happen to be so locatedrelative to the flash tube or tubes that very little of the flash tubelight strikes the end reflectors, the protective end caps last describedmay not be required.

The size of the opening 36a, as will be clear from the drawing, is madeat least large enough to allow a collective lens such as lens 38disposed in axially aligned relation to the fiber 26 to focus upon theend of the fiber in such a manner as to collect substantially all of theradiation emitted therefrom and direct the collected radiation toward asecond lens or lens system such as at 40 which thereafter directs theradiation to a suitable means such as a detector or the like indicatedat 42. This detection means for the laser component just described maybe a photo-multiplier provided with a suitable infra-red (IR) filter 44in the path of the radiation.

When the flash tube 14 as the pump power source is triggered by the highvoltage pulse, it will be appreciated that yellow and blue light whichis principally emitted from the flash tube travels in substantially alldirections towards the laser component 12. Since the cylindricalcladding 28 of the laser has an index of refraction slightly less thanthe neodymium barium crown glass forming the fiber 26, it serves as anoptical means for collecting and concentrating the pumping light moreefficiently upon the thin long laser glass fiber therewithin therebyeffecting a decided improvement in efficiency (which is substantiallyproportional to the index of refraction of the cladding) over that whichwould otherwise be obtained if only an unclad laser glass fiber werebeing employed. Also, other factors such as structural strength,concentric alignment, ease of handling, supporting, coating etc. of sucha thin fiber should be considered and, for this reason, even though anunclad laser fiber of barium crown glass with neodymium may be used insuch an assembly and made to laser, a clad fiber is preferred.

Barium crown glasses with varying percentages of neodymium therein andwhich are set forth herein as illustrative of one form of applicant'sinvention have been made and used to form a laser fiber cavity with andwithout cladding thereon. These have been used, for example, with aGeneral Electric FT524 flash tube operating from a condenser bankproviding 80 microfarads charged to a minimum power of 2.5 kilovolts toproduce laser oscillations. With another flash tube (an E.G. & G. tubeNo. FX33) and a power input of about 25 watt seconds, laser emissionhave also been obtained. Concentrations of neodymium 0.1, 0.25, 0.50,1.0 and 2.0% by weight were used in barium crown glasses with successfullasering results. Laser emission took place at approximately 1.06microns. For the 2% concentration, there was no indication ofconcentration quenching. This was established by measuring the lifetimefor the fluorescent emission for each of these glasses of differentconcentrations and finding that they were the same in all cases. Thisindicated that there was no loss of quantum efficiency at the higherconcentrations.

The batch composition of the barium crown glass having 2% by weight ofneodymium and mentioned previously as an example of a laser glass forforming the fiber 26 contains the following ingredients:

    ______________________________________                                        Silica (SiO.sub.2)        237.6 units                                         Potassium Carbonate (K.sub.2 CO.sub.3.11/2 H.sub.2 O)                                                   107.8 units                                         Barium Carbonate (BaCO.sub.3)                                                                            68.1 units                                         Barium Nitrate (Ba(NO.sub.3).sub.2)                                                                      40.8 units                                         Barium Hydroxide (Ba(OH).sub.2 .8H.sub.2 O)                                                              49.4 units                                         Antimony Oxide (Sb.sub.2 O.sub.3)                                                                        4.0 units                                          Neodymium Oxide (Nd.sub.2 O.sub.3)                                                                       10.0 units                                         ______________________________________                                    

This laser glass was prepared by placing the batch material in a claypot in a globar electric furnace using a filling time of two hours at atemperature of approximately 2600° F. and then elevating the temperatureof the melt to approximately 2700° F. and stirring for about one hourbefore lowering the temperature of the glass to approximately 2550° F.where it was maintained for a period of about five hours before casting.The size of the above batch was one pound and, of course, the preferredtemperature at casting is a function of the size of the cast. No specialatmosphere was necessary in the furnace.

The percent oxide composition by weight as calculated from this batchwould be as follows:

    ______________________________________                                        Silica (SiO.sub.2)        57.4%                                               Potassium Oxide (K.sub.2 O)                                                                             14.9%                                               Barium Oxide (BaO)        24.3%                                               Antimony Oxide (Sb.sub.2 O.sub.3)                                                                        1.0%                                               Neodymium Oxide (Nd.sub.2 O.sub.3)                                                                       2.4%                                               ______________________________________                                    

These percentages will be a very close approximation of what a chemicalanalysis of this glass would give.

With a concentration of 0.1% by weight of neodymium in a similar bariumcrown glass, a near minimum condition for lasering action was obtainedwhile using the FT524 flash tube operating from a condenser bank of 320microfarads charged to 5 kilovolts.

For another group of glasses, with different concentrations, it wasfound that for a 10% concentration of Nd₂ O₃, there was a shortening ofthe lifetime for the fluorescent emission thereby indicating someconcentration quenching.

Hence, the upper limit of concentration of the neodymium in the bariumcrown glass for forming the laser is in the order of possibly 30%. Onthe other hand, the lower limit, if an optical system of high efficiencyis employed for coupling the pump light to the fiber would providelasering at a neodymium concentration as small as 0.10% or less. Underoptimum conditions, a concentration of as little as 0.01% by weight ofneodymium oxide will laser. Thus, I now believe that the limits for theconcentration of the neodymium are between 0.1% and approximately 30% byweight.

While good efficiency as roughly determined by the lifetime of thefluorescence is indicated for the barium crown glass with neodymium,most silicate glasses are suitable as a laser glass and a furtherexample of such silicate glass is soda-lime base glass produced byAmerican Optical Company and sold commercially under the AO Crown and towhich like amounts of neodymium has been added as the active material.Another glass of good Q value when combined with neodymium is a 15% byweight lead glass. Other inorganic base glasses into which neodymium orother of the active materials have been added and examined for quantumefficiency as determined by their lifetimes for fluorescence emissionare oxide base glasses formed of aluminum zinc phosphate, sodium borate,15% lead silicate, high index of refraction lead silicate, high silicaborosilicate glasses and vitreous germanate glass. Fluoride base glassessuch as beryllium fluoride can also be used. The indices of refractionand dispersion of the above inorganic base glasses are of knownestablished commercial values but would vary somewhat when doped withselected amounts of the active laser materials. For the above bariumcrown glass with 2% neodymium, the index of refraction is 1.5427 and itsdispersive value is 59.4.

It is well to keep in mind that the base glass to which the active lasermaterial is to be added should be non-absorbing and non-scattering tothe wave lengths of interest which are the pumping lines for the activeions being used and the wave length at which laser emission takes place.

Neodymium has already been mentioned as an active material or lasermaterial with the fiber or core glass 26, and is in a trivalent form;other active materials include Samarium⁺ ⁺ , Samarium⁺ ⁺ ⁺ , Europium⁺ ⁺, Europium⁺ ⁺ ⁺ , Uranium⁺ ⁺ ⁺ , Terbium⁺ ⁺ ⁺ , Holmium⁺ ⁺ ⁺ , Erbium⁺ ⁺⁺ , Thulium⁺ ⁺ ⁺ , Dyprosium⁺ ⁺ ⁺ , Ytterbium⁺ ⁺ ⁺ , Cerium⁺ ⁺ ⁺ , andPraseodymium⁺ ⁺ ⁺ . These materials, it is noted, are rare earthelements with the exception of uranium. These active materials may beused separately or in various combinations in the laser base glass.

An important advantage derived from the use of a cladding 28 upon thefiber laser core 26 was previously mentioned but merits repetition. Inorder to have a laser oscillatory structure, it is necessary to have ahigh Q cavity and this requires that means be provided for obtaining acondition whereby well defined standing waves be established across atleast a part of the cross-section of the laser component. Since a singleunclad glass sample of good optical quality and free from striae is hardto provide, it is an easier matter to employ a fiber of smallcross-sectional size and use a cladding of greater thickness and lowerrefractive index, so that the cladding will act as a light pipe or waveguide structure improving the optical properties thereof from the pointof view of a high Q cavity for laser oscillations. Also, it is possibleto obtain the surface wave type of mode previously mentioned with highvalues of quality factor or Q. Furthermore, since the fiber is ofrelatively small cross-sectional size compared to the cladding and ifthe cladding is compatible therewith so as to give an interface of goodoptical quality, it is more likely that the entire length of the lasercomponent will go into laser oscillation and thereby improve tecoherence properties of the emitted beam.

While laser action has been obtained with a clad fiber of 0.015 inchesfor the fiber diameter, as mentioned previously, a lasering fiber ofstill smaller diameter may be used to arrive at a condition wherein onlyone or a few clearly distinguishable progagation modes can exist thereinfor reasons explained above. A fiber diameter of as small as 5 micronsand a cladding thereon of glass of a lower index of refraction and of aslittle as 1 mircon in thickness can be used. The upper limit ofthickness of such a cladding is not critical, but its thickness shouldnot be so great as substantially to attenuate the pumping light.

The conditions of laser oscillation in the visible and infra-red regionsof the spectrum are well established. The fundamental requirements,however, are that the laser material be capable of fluorescing and thatan inversion in population take place between the two different energylevels between which the fluoroscent emission takes place. To fulfillthe latter requirement, in the case of a glass laser, it requires thatthere also be fairly strong absorption of the pumping energy to permitpumping action by the light source.

While various rare earth and other laser materials have been mentioned,for a more detailed examination of the energy levels of such materialsreference is made to D.S. McClure, "Electronic Spectra of Molecules ofIons in Crystals", Part II (Spectra of Zons in Crystals) SOLID STATEPHYSICS, No. 9, page 399 (1959). Attention is called at this time to theenergy levels given in FIG. 9 for the neodymium triple plus ion whichare plotted against wave numbers.

The use of organic glasses as the base glass into which the lasermaterial is put have already been mentioned. The organic glasses includecommercially available plastics such as methyl methacrylate, orplexiglass, acrylics and vinyl chlorides which are solids at roomtemperature as well as sugars which have been melted to get rid of theircrystalline structure and then cooled to form a glass. Also, glasseswhich at room temperatures are liquids and thus have to be cooled toobtain the glassy state and maintained at low temperatures by such meansas liquid nitrogen, liquid helium or the like. An example would beether-isopentane-alcohol cooled to approximately 77° Kelvin. The indicesof refraction and dispersion of the above organic glasses are also ofknown values.

The rare earth material may be put into any of the organic glasses asfree ions or as compounds of the active material such as neodymiumacetate, neodymium chloride or a neodymium chelate dissolved in theglass.

In the selection of the light source, such as the flash tube 14 of FIG.1 for pumping the laser oscillator or the laser amplifier, considerationshould be given to both emission characteristics and the intensitieswhich can be provided. The more efficient overall operation can beachieved if the pumping light source has its energy concentrated in theabsorption bands of the laser material being used. The absorptionspectra for neodymium⁺ ⁺ ⁺ is given in FIG. 10 and it will be seen thatit has a strong absorption band in the yellow at 5800 Angstroms (i.e.580 millimicrons). There are also absorption bands at 5300A, at 3550A inthe ultra-violet and several in the near infrared. In order to make theglass fiber laser, it is necessary to absorb a sufficient amount of thispumping light to produce an inversion of population in the lasermaterial. Thus, a light source must be used that has sufficient energyin the absorption bands of the laser material.

A mercury vapor lamp is well adapted for this use since it has a strongemission line at 5790 Angstroms, or a sodium vapor lamp may be usedsince it has strong lines at 5890A and 5895A. Also, as the pumpinglight, use may be made of a carbon are whose electrodes are doped by theneodymium or rare earth whose spectrum lines coincide with theabsorption bands of the laser material. With a flash tube such as inFIG. 1, the emission is of pulsed nature and accordingly is notcontinuous with time. Therefore, for CW or continuous wave laseroperation, a continuous light source with enough intensity must be usedand with good optical coupling being provided to get the pumping lightinto the sample. Means may be needed in continuous wave laserapplications to carry heat away from the laser component since excessivetemperature rise can impair the lasering action desired. For absorptionin the yellow, 40% of the energy within the lasor element is convertedinto heat; for absorption in the ultra-violet at 3500A, about 70% of thelight energy within the laser element is converted to heat.

It can be seen from these conditions that the laser rod or componentmust be of a proper thickness or diameter. When it is too thick, thelight energy does not get to the center of the laser material. It getsabsorbed before it goes that far. Also, too much heat in continuous waveoperation might be generated without being carried away properly. Thus,it is desirable to have the diameter of the laser fiber small, whetherclad or unclad, so as to improve the surface-to-volume ratio thereof. Ina clad fiber, the core of active material can be a very smallcross-section but because of the cladding, the diameter of the wholeunit could be sufficiently large to permit easy handling.

In FIG. 9, an Energy Level Chart for the neodymium triple plus ion isshown. The chart is described in detail in the following reference: E.H. Carlson and G. H. Dieke, "The State Of the Nd³ ⁺ Ion As Derived FromThe Absorption And Fluorescence Spectra of NdCl₃ And Their ZeemanEffects", JOURNAL OF CHEMISTRY AND PHYSICS, 34 1602 (1961). This energylevel diagram is for small quantities of neodymium doped in lanthanumchloride. Although this diagram is for the chloride, it is substantiallythe same for the neodymium glass.

The laser energy is pumped from ground level, indicated in this figureat 120, to higher atomic levels and principally to levels D and L asindicated by horizontal lines 122 and 124 and by pumping lines aa andbb. At this time, the atoms of the material are in a highly excitedstate. They then give off some of this energy as heat while droppingdown or making a non-radiative transition to a lower energy level Rindicated at 126. The transition from this energy level to the threelower levels X, Y and Z as indicated by lines cc, dd, and ee isaccompanied by strong emissions.

In FIG. 8, an emission spectrum or fluorescent spectrum such as obtainedwith and without laser action of the neodymium in the glass is indicatedby dotted line AA and solid line BB respectively. In this figure,relative intensities from 0 to 10 are indicated in a vertical directionand wave lengths (in millimicrons) are plotted in the horizontaldirection thereof.

Since the cross-sectional size of the glass fiber for laser purposes canbe exceedingly small, and even though a clear glass cladding of lowerrefractive index and of very small size is used, it is possible tosecure a multiplicity of such fibers together in closely bunchedrelation so that these fibers may be operated in phase. In such a case,the multifiber group so formed would have its opposite ends ground andpolished so as to be optically flat and normal to the axis at the end ofthe bundle. The opposite ends of the bundle would then be silvered orotherwise coated for high reflectivity and, as in the case of the rod 12of FIG. 1, would have one of these reflective coatings of such athickness as to provide approximately a 2% light transmission.

In FIG. 3 is shown a modified form of laser assembly 46 employing alaser component 48 much like that of FIGS. 1 and 2 in that it iscomprised by a thin, long lasser fiber formed of glass with a glasscladding of lower refractive index thereon, and has reflecting endsurfaces 48a and 48b of the type described in connection with FIGS. 1and 2. Thus when a pumping action of the laser component is taking placewithin the cavity thus formed, light is transmitted through the 2%transmissive coated end 48b thereof and enters an optical system or thelike generally indicated by the numeral 50. In this modifiedconstruction, a plurality of spaced and parallel-energized flash tubes52 are employed at circumferentially spaced locations about the lasercomponent, and outwardly of the flash tubes and in concentric relationto the laser component is disposed a metallic reflector 54 forconcentrating pumping light onto the glass fiber core of the component.Advantage may be taken of the fact that the reflector 54 is made ofmetal by connecting a high voltage triggering-potential lead 56 directlyto the reflector. Conductors 58 are shown for supplying pump power tothe bank of parallel connected flash tubes 52.

In FIG. 4 is shown a different form of construction 58 which is intendedto function as a travelling wave type of lasser amplifier. In thisconstruction, a pumping light source is indicated at 60 and around thislight source and in concentric relation thereto is disposed acylindrically-shaped transparent tube 62 about which is coiled, in turn,a thin, long flexible glass laser component 64. This component comprisesa laser fiber formed of glass and a clear glass cladding of lowerrefractive index and accordingly provides wave-guide mode propagationfor reasons previously explained. This filament-like component is ofsuch length and cross-sectional dimensions as to be readily coiledaround the tube 62 a large predetermined number of times. Thearrangement just described is such that when pumping energy is providedby the light source 60 during transmission of an optical signal from endto end through the laser component 64, as indicated by the arrow 65,energy will be absorbed by the laser fiber in such a manner that energybuilds up in and is then emitted by the coiled filament-like component64 so as to amplify this optical signal as it travels from the input endto the output end 66 of the fiber. In such an arrangement, the signalgets boosted in intensity, for example, to an amount sufficient tooffset any attenuation thereof during a long distance trasmission of theoptical signal through a long length of clear fiber before reaching theinput end of this travelling wave type of booster assembly. It will benoted that the pumping light enters the fiber through the side wallsthereof, while the increase in signal strength is due to the absorptionof energy from this pumping light by the laser component and theemission of this absorbed energy under stimulation of the signaltravelling by wave guide mode progagation along the length of the laserfiber.

In order to get wave guide mode propagation, as the mode in which thelaser oscillations build up or laser amplication takes place, it isnecessary as previously explained for the fiber core to have a higherindex of refraction than the index for the cladding. However, becausethe field distribution in a wave guide mode penetrates into the claddingas pointed out above, it is possible to incorporate the active neodymiumions into the cladding and to have the core made of a clear undopedglass. Under these conditions, some degree of mode selection is providedfor the modes whose fields penetrate the most into the cladding and areaccordingly the ones to be excited by laser action in the cladding.

In FIG. 5, a modified construction is shown at 70 and instead of beingsuch as to act as a signal amplifier, the construction is such as toserve more in the sense of a resonant cavity laser. The similar thin,long flexible laser component is shown at 72 and comprises a laser fiberof glass which is clad with clear lower-index glass, as before, andwound in similar manner upon a transparent cylindrical tube 74. Thistube is in concentric relation to the pumping flash tube 76, as before,and the arrangement is such as to provide pumping for fluoroescent laseremission from one end of the fiber as indicated by the arrow 78.However, in this case, the opposite end of the fiber, as indicated at80, will be optically finished and coated with a capped reflectivecoating in the same manner as the non-transmitting end of the fiber inFIG. 1. The great length of this laser component enables a build-up ofenergy in the fiber.

While not shown for reasons of simplicity, the structures of FIGS. 4 and5 may utilize a concentric external light reflector arranged similar toand having the same light concentrating purpose and function as thereflector 54 of the FIG. 3 construction.

A different modified form of resonant cavity laser component isindicated in FIG. 6. In this case, the component is formed by a hollowcylindrically-shaped thin, long element 82 formed of a glass lasermaterial. A core of clear soda-lime glass is contained at 84 within thishollow cylindrical member and in good optical contact therewith, and asimilar glass as a cladding is shown at 86 surrounding the laser glasselement. Both the core 84 and cladding 86 have a lower refractive indexthan does the laser glass. The opposite ends of thiscylindrically-shaped laser component are optically finished so as to beparallel to each other and are coated with a reflective layer, such assilver, as indicated at 88 and 90, but with one of these two coatingsbeing applied under carefully controlled conditions so as to haveapproximately a 2% light transmission for the fluorescent emissionprovided during lasering in a manner similar to that described abovewith reference to FIGS. 1 and 2.

In FIG. 7 is indicated a fiber optical communication line 92 disposedwithin an outer protective shield 94, and at spaced locations therealongmay be disposed a laser amplifier as indicated generally at 96 forboosting an optical signal travelling in line 92 as suggested by arrow97. This arrangement enables attenuation of such a signal to beminimized or avoided when transmission is taking place over extendeddistances. Pumping light sources 98 are diagrammatically indicatedwithin the amplifier housing 96, and it will be apparent that a laserfiber amplifier arrangement disposed therewithin may very well be thecoiled fiber laser type described above so as to absorb pumping lightenergy as a travelling wave structure for strengthening or amplifyingthe optical signal traversing the line 92.

A laser component or rod of a geometry quite similar to that shown inFIG. 6 can be obtained by having a single small diameter laser fiber 130placed eccentrically relative to its cladding glass 132 as indicated inFIG. 11. Both the geometry shown in FIG. 11 and the geometry shown inFIG. 6 are of importance in pumping arrangements in which the pumpinglight enters the rod through the end of the rod from substantially alldirections. A principal advantage of these geometrics, when used withend pumping light, is that the skew rays internally propagated down therod are more readily intercepted by the laser element than would be thecase if the active laser glass were in the center of the rod. Theprecise positioning of the active laser fiber 130 relative to the axisof the cladding rod 132 and their relative transverse dimensions are amatter of detailed design considerations, depending upon the end resultsdesired.

In addition to the use of pumping light sources that have strong linesat the same wave length regions in which the neodymium or other selectedlaser material absorbs pumping light energy, one can also incorporate inor place near the laser material a fluorescent material which serves toconvert other wave length regions of the pumping light source intodesired ones effective to pump the laser material. For example, most ofthe intensity of a high pressure mercury lamp is in the ultra-violet. Ifthe fluroescent material last mentioned were incorporated in closeproximity to a high-pressure mercury lamp and were positioned orarranged to illuminate the laser material and if, furthermore, thefluorescent material had the property that it would with a high quantamefficiency fluroesce in the yellow after absorbing ultra-violet light,then this would provide a suitable fluorescent converter type of pumpinglight system. Examples of such fluorescent materials are uranylcompounds including glasses and various commercially availablefluorescent plastics.

Suitable materials incorporated into laser glasses or compounds of theactive ions, such as recited just above, can have the property thatabsorption of pumping energy may take place by way of the other materialor parts of the compound of the rare earth and a subsequent transfer ofenergy may then take place directly to the laser ion without the needfor the emission of light energy and re-absorption of the emitted lightenergy by the lasering rare earth. An example is when uranium andsamarium are incorporated into calcium oxide; some light is absorbed bythe uranium ion, and a non-radiative transfer of energy is made to thesamarium ion. Another example is the absorption of light by the ligandof a chelate and then the subsequent non-radiative transfer of energy toa rare earth ion effective to provide lasering action.

A laser component of modified form and its associated energizingstructure, providing a character of laser operation analogous to thatlast described, is illustrated in FIGS. 12 and 12a. The laser component136 is of elongated hollow tubular construction presently to bedescribed more fully, and is helically wound upon a metal cylinder 137.This helically would laser component is covered by a metal cylinder 138.An enlarged cross-sectional view of the laser component 136 isillustrated in FIG. 12a, and it will be seen that this componentincludes a hollow glass tubing 139 of given index of refraction n₁ and aconcentric enclosing glass cladding 140 of smaller index of refractionn₂. The hollow interior of the tubing 139 is filled with a mixture ofhelium and neon gases which, of course, have unit value of index ofrefraction thus to establish appropriate boundary conditions for waveguide mode propagation within the tubing 139 by reason of the lowervalue of index of refraction of the cladding 140. In particular, thetubing 139 is filled with helium at a pressure of approximately onemillimeter of mercury and with neon at a pressure of approximately1/10th millimeter of mercury. The cladding 140 may by way of examplehave an external diameter of 100 microns, and the tubing 139 may in thisexample have an internal diameter of the microns with a wall thicknessof one micron. The metal cylinders 137 and 138 are connected to inputterminals 142 which are energised from a source of radio frequencypotential, the source having a frequency of the order of 30 megacyclesand a power output of the order of 50 watts.

The radio frequency energization applied between the cylinders 137 and138 of the FIG. 12 structure causes the helium gas to absorb energy byelectron excitation, and this absorbed energy is then transferred by aresonant form of action to the neon gas to effect an inversion ofpopulation between two energy levels of the latter. Thus the helium gastransfers energy from the input radio frequency excitation to the neongas, with subsequent radiative energy transition two energy levels bythe latter to provide laser action. The laser component 136 ishermetically sealed at its ends as minute glass dises or the like fusedthereto, and the end surfaces may be polished to provide a lightamplifier or one or both of the polished ends may be provided withreflective silvered end coatings as previously described in connectionwith FIGS. 1 and 2 to provide a resonant cavity structure. It will beunderstood that the particular cross-sectional parameters of the lasercomponent 136 and its operating temperature, for example by immersion ina liquified gas at low temperature, are selected according to aparticular desired application as will be well understood by thoseskilled in this art.

While specific forms of invention have been described for purposes ofillustration, it is contemplated that numerous changes may be madewithout departing from the spirit of the invention.

I claim:
 1. A laserable material consisting essentially of a plastichost having a chelate of a rare earth metal dispersed in solid solutiontherein, which rare earth metal in relation to the prevailing laseremisive light absorptive characteristics of said host supports in saidhost a sufficient inversion in population between two energy levels ofsaid rare earth metal as to provide at the wavelength of stimulatedemission of said rare earth metal, light energy by stimulated emissionin excess of all light energy losses in said laserable material.
 2. Anactive transmission line for the guidance of electromagnetic wave energycomprising:an elongated transparent dielectric material having aneffective refractive index which decreases from a maximum at the centerto a minimum at the outer edge of said line; characterized in that saidmaterial includes therein active laser material suitable for pumping andsubsequent stimulated emission at the frequency of said electromagneticwave energy; and means, including an elctroluminescent materialdistributed along said line for pumping longitudinally spaced regions ofsaid dielectric material.
 3. The transmission line according to claim 1wherein said laser material is longitudinally distributed along all ofsaid line.
 4. The transmission line according to claim 1 wherein saidlaser material is longitudinally distributed along selected regions ofsaid line.
 5. The transmission line according to claim 1 wherein saidelectroluminescent material is separate from said line.
 6. An activetransmission line for the guidance of electromagnetic wave energycomprising:an elongated transparent dielectric material having aneffective refractive index which decreases from a maximum at the centerto a minimum at the outer edge of said line; an active laser material,suitable for pumping and subsequent stimulated emission at the frequencyof said electromagnetic wave energy, transversely distributed onlywithin the inner portion of the cross-sectional area of said line; andmeans distributed along said line for pumping longitudinally spacedregions of said dielectric material.
 7. An active transmission line forthe guidance of electromagnetic wave energy comprising:an elongatedtransparent dielectric material having an effective refractive indexwhich decreases from a maximum at the center to a minimum at the outeredge of said line; an active laser material, suitable for pumping andsubsequent stimulated emission at the frequency of said electromagneticwave energy, transversely distributed throughout the cross-sectionalarea of said line; and means distributed along said line for pumpinglongitudinally spaced regions of said dielectric material.